This way depends on an XHTML document parse tree, which means in plain English that it goes through your markup and makes sure you’ve set things up properly. If you take away this step, the validator doesn’t know it’s supposed to have a screen media type. So if you try validating just the CSS minus a containing document:

Then you end up side-stepping the issue, and your CSS gets the thumbs-up. So I’ve done just this in the footer way down yonder to stem the tide of e-mail complaining about my so-called ‘invalid CSS’. A hack? Maybe. Kinda. Not really. I’m still validating my CSS, only without the surrounding context. I should be providing that context, but the validator should tell me it’s valid. I’m just trying to make it all gel, baby.